From the state of the art, hydrodynamic clutches in start-up units for vehicles are known in a multitude of implementations. The start-up unit comprises thereby a drive that can be coupled to an input and an output that can be coupled to a power take-off. The hydrodynamic clutch which comprises a primary impeller and a secondary impeller, that together form a toroidal working chamber, is arranged between the input and the output. The primary impeller is thereby, for example, provided with a so-called primary impeller cup that is torque proof connected to it, and that encloses the secondary impeller in the axial direction, and completely in the circumferential direction. In addition, the start-up unit comprises a controllable clutch in the form of a direct-drive clutch that is arranged parallel to the hydrodynamic components, in particular the clutch, and that can be controlled together with it or on its own. This means that through both clutches two power branches are created, whereby the power flux either takes place only through one of the clutches, or collectively through both. The controllable clutch comprises thereby at least a clutch input element and a clutch output element, whereby the clutch output element is coupled, at least indirectly torque proof, to the secondary impeller. The input clutch element is connected, at least indirectly torque proof, with the primary impeller and/or the input. The means for the creation of a frictional contact between the individual clutch elements comprise thereby a piston element that can be impacted upon by a pressure medium. It can be arranged separately from the clutch disks or else formed directly on the secondary impeller in particularly compact implementations. The hydrodynamic clutch is in addition provided with a utilities supply system. The clutch can thereby be flowed through centrifugally and centripetally. In the case of centripetal flow-through, the utilities are led, via the utilities supply channel, along the outer circumference of the secondary impeller and injected into it in the radial direction in the region of the outer circumference of the toroidal working chamber. The force created by the utilities is thereby utilized to keep the controllable clutch in a relaxed state or to operate with at least a certain amount of slippage, respectively. The exit from the toroidal shaped working chamber thereby takes place in the region of the radial inner diameter of the working chamber in a space that lies thereunder and which is also designated as a second utilities guide channel or chamber. The first utilities channel and/or chamber, that is restricted by the inner circumference of the casing and the outer diameter of the secondary impeller, as well as the second utilities supply channel and/or chamber can thereby be interchanged as regards their function. This is necessary in particular during the change over from centripetal to centrifugal flow-through. The supply of the utilities for the hydrodynamic coupling takes place via the second utilities supply channel and/or chamber in the region of the radial inner diameter of the working chamber, whereby the exit takes place in the region of the radial outer diameter of the working chamber on one of the impellers or between both. The controllable clutch is then activated. In such start-up units the performance ratio can thus be varied via the individual clutches—controllable clutch or hydrodynamic clutch. It is thereby in particular during the operation of the hydrodynamic clutch desirable that, for the avoidance of a negative retroaction on the number of revolutions of the driving machine, the torque that can be picked-up by the hydrodynamic clutch and that corresponds to the torque that can be picked-up by the primary impeller, is kept as low as possible. This is attempted by adjustment of a minimal fill factor. However, it has become clear that this measure only is insufficient since exactly in the range of very high coupling slippage, for example of 70 to 100 percent, torques that are too high are still picked-up by the clutch. It can lead therefore to an undesirable retroaction in the form of suppression of the number of revolutions of the driving machine that is coupled to the hydrodynamic clutch, so that then the desired driving dynamics is no longer present.